This invention is in the field of semiconductor photovoltaic detectors. Such detectors include those having a semiconductive chip or slab, one side of which is exposed to radiation to be detected. The other side may be covered with an insulating layer, and with a conductive layer atop the insulating layer. A voltage source connected between the chip and the conductive layer biases the chips for electron emission. Such detectors are known in the art as photocathodes. The insulating layer is usually relatively thick, because thin insulating layers are prone to having pinholes. Such pinholes allow the conductive coating (when applied) to contact the chips, and such contact will degrade the electron image on the chips. A thick insulating layer, however, limits the frequency response or recovery time of the detector. Our invention is able to resolve the mutually contradictory requirements for the insulating layer.